1. Field of the Invention
This invention relates to scale abatement and, more specifically, this invention relates to a method and apparatus for substantially reducing or eliminating the rate and extent of formation of calcium carbonate and magnesium hydroxide scale in water distillation equipment.
2. Brief Description of the Prior Art
Much research has been directed to the elimination of alkaline scale formation during distillation of water which contains dissolved salts of magnesium and/or calcium, such as brackish water or seawater. In all types of seawater distillation equipment, heating of seawater to a temperature up to about 180.degree. F. produces scale which is predominantly calcium carbonate (CaCO.sub.3). At temperatures about about 200.degree. F., magnesium hydroxide [Mg(OH).sub.2 ] scale predominates. At temperatures between about 180.degree. F. and 200.degree. F., either type of scale, or mixtures thereof, may be encountered. Calcium carbonate and magnesium hydroxide scales are collectively referred to a alkaline scales.
Since scale is a heat insulator, even a thin layer of scale on heat transfer surfaces or other components of process equipment significantly reduces the heat transfer capability of the components. Accumulation of scale on evaporator tubes may result in significant reductions in throughput capacity, increases in energy input per unit of throughput capacity, or both. Scale accumulation results in frequent shutdowns, the cost of which is substantial, as is the direct cost of cleaning scaled tubes.
Various means have been utilized in attempts to reduce or eliminate scale formation in seawater distilling units, with limited success. In attempts to destroy total alkalinity (the sum of bicarbonate (HCO.sub.3.sup.-), carbonate (CO.sub.3.sup.-2) and hydroxyl (OH.sup.-)ions) in evaporator brine, acids, generally mineral acids, have been added to feed streams in distillation units. However, effective control of scale formation by continuous acid addition is difficult to attain due to the highly corrosive nature of mineral acids and the sensitivity of scaling reactions to acid concentration. Precise control of acid dosing is required, since introduction of excess acid into a seawater feed stream results in equipment corrosion, while introduction of insufficient acid results in rapid scale formation. Continuous acid addition is a costly process and, unless carbon dioxide formed in the chemical reaction between seawater alkalinity and acid is substantially completely removed, as by deaeration, rapid rates of corrosion of copper alloy and steel evaporator components result. This is impractical in many seawater distillation installations.
The use of inorganic or organic chemical scale control additives to prevent alkaline scale formation has met with only limited success, and is often not effective at relatively high brine temperatures. Thus, while many commercially available scale control additives provide good control of calcium carbonate scale formation, such chemicals have been only marginally effective in controlling magnesium hydroxide scale formation at relatively high temperatures.
It has long been recognized that liberation of carbon dioxide from brine as a result of boiling promotes decomposition of bicarbonate and carbonate ions in the brine, ultimately resulting in the formation of hydroxyl ions and insoluble magnesium hydroxide scale at high temperatures. Thus, attempts have been made to inhibit hydroxyl ion formation by the addition of pressurized carbon dioxide to brine being heated under nonboiling conditions, such as in multistage flash evaporators, in order to inhibit the hydroxyl ion formation reaction.
Although this has achieved some success on a pilot scale in multistage flash evaporator units wherein brine is heated under pressure and flashed in a chamber separate from the heating stage, this approach has not been successful in other types of evaporator units in which boiling brine directly contacts heat transfer surfaces. Further, the presence of free (chemically uncombined) carbon dioxide promotes corrosion, and has made carbon dioxide addition commercially impractical in all types of evaporating apparatus.
Checkovich U.S. Pat. No. 3,218,241 (Nov. 16, 1965), the disclosure of which is hereby incorporated by reference, describes a method of controlling scale formation in multi-stage flash (MSF) fresh water recovery systems by maintaining the concentration of carbon dioxide in pressurized brine being heated at a level sufficient to inhibit the hydrolysis of bicarbonate ions to carbonate ions. The Checkovich patent states that this may be accomplished by recycling carbon dioxide released during distillation of the feed seawater.
The Checkovich patent teaches that formation of scale can be retarded by addition of glassy phosphates or other chemicals having chelating or wetting properties. Addition of acids may be required at high temperatures.
However, the disclosed method of Checkovich No. 3,218,241 requires a relatively low pH (e.g. 7.5 or below) and extremely high free carbon dioxide concentrations (e.g. 4-15 ppm). Such conditions accomplish scale abatement only at the cost of relatively high corrosion rates of commonly used steel and copper alloy process equipment components. As a result, this approach to the problem of controlling alkaline scale formation has seen little or no commercial use.
Summarizing, the problem of controlling alkaline scale formation in commercial seawater distilling plants has been approached in several ways. Plants which are operated at relatively low temperatures (i.e. below about 190.degree. F.) may use the addition of polyphosphates or other chemical scale control additives to control the formation of calcium carbonate scale. Operation above about 190.degree. F. has required continuous acid addition to feedwater followed by deaeration to destroy the alkalinity of feedwater and to remove carbon dioxide from the system.
Alternatively, continuous addition of a chemical scale control additive has been combined with mechanical means (e.g. sponge rubber balls of the Taprogge type) for removing soft scale from heat transfer surfaces, or with partial destruction of feedwater alkalinity by continuous acid addition followed by deaeration for carbon dioxide removal.
Prior systems generally have required periodic shutdown with circulation of acid to remove accumulated scale. The frequency of shutdown has been inversely related to the effectiveness of the chemical and/or mechanical treatment employed. In general, an attempt is made to strike a balance between the cost, complexity and corrosion risks of the method of scale control pretreatment employed, and the cost, lost production and corrosion associated with shutdown and periodic acid cleaning.